Allen was the first American journalist to report on the massacre of Badajoz. On 23 August 1936, nine days after the fall of the city, he entered Badajoz and saw the executions taking place.[3] On 30 August the Chicago Tribune published its report on the massacre: “Slaughter of 4,000 at Badajoz, city of horrors”,[1] the first report about it in the United States. As a result, the nationals put a price on his head and the supporters of the Nationals in the States started a campaign against Allen.[3] In November 1936 he interviewed the chief of Spanish falange José Antonio Primo de Rivera who was imprisoned in Valencia. It was the last interview before his execution. After that, Allen returned to the United States.

In 1938, Allen was hired, then quickly fired, by the nascent left-leaning avant-garde magazine Ken.[4] In 1940 he went to France as a correspondent for the North American Newspaper Alliance. There he helped to organize a group to help French artists to flee the country. Because of this the Gestapo arrested him in 1941, although he was later freed. In 1942 he took part in the campaign of North Africa. After that, he returned to America and worked for various newspapers until his death in 1972.